One little known side effect of test_bit is that it adds
a kind of a compiler barrier since the pointer parameter
is volatile.
It seems risky to change the semantics of test_bit so let's just
add __test_bit (matching __set_bit and __clear_bit) that does
not add such a barrier.
Will be used by kvm on x86, where it shaves several bytes off
the binary size. Small win, but comes at no cost, so why not.
Signed-off-by: Michael S. Tsirkin <m...@redhat.com>
---
x86 maintainers - please specify whether you are ok with
adding this to arch/x86/include/asm/bitops.h
An alternative is to add this to kvm/x86 only.
It might be worth it to add this to all architectures,
though I haven't explored too much.
arch/x86/include/asm/bitops.h | 30 ++++++++++++++++++++++++++++++
1 file changed, 30 insertions(+)
diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h
index cfe3b95..9229334 100644
--- a/arch/x86/include/asm/bitops.h
+++ b/arch/x86/include/asm/bitops.h
@@ -323,6 +323,24 @@ static inline int variable_test_bit(long nr, volatile
const unsigned long *addr)
return oldbit;
}
+static __always_inline int __constant_test_bit(long nr, const unsigned long
*addr)
+{
+ return ((1UL << (nr & (BITS_PER_LONG-1))) &
+ (addr[nr >> _BITOPS_LONG_SHIFT])) != 0;
+}
+
+static inline int __variable_test_bit(long nr, const unsigned long *addr)
+{
+ int oldbit;
+
+ asm volatile("bt %2,%1\n\t"
+ "sbb %0,%0"
+ : "=r" (oldbit)
+ : "m" (*addr), "Ir" (nr));
+
+ return oldbit;
+}
+
#if 0 /* Fool kernel-doc since it doesn't do macros yet */
/**
* test_bit - Determine whether a bit is set
@@ -330,6 +348,13 @@ static inline int variable_test_bit(long nr, volatile
const unsigned long *addr)
* @addr: Address to start counting from
*/
static int test_bit(int nr, const volatile unsigned long *addr);
+
+/**
+ * __test_bit - Determine whether a bit is set
+ * @nr: bit number to test
+ * @addr: Address to start counting from
+ */
+static int __test_bit(int nr, const volatile unsigned long *addr);
#endif
#define test_bit(nr, addr) \
@@ -337,6 +362,11 @@ static int test_bit(int nr, const volatile unsigned long
*addr);
? constant_test_bit((nr), (addr)) \
: variable_test_bit((nr), (addr)))
+#define __test_bit(nr, addr) \
+ (__builtin_constant_p((nr)) \
+ ? __constant_test_bit((nr), (addr)) \
+ : __variable_test_bit((nr), (addr)))
+
/**
* __ffs - find first set bit in word
* @word: The word to search
--
MST
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
